Arrays of single living cells have been made by inserting individual cells into individual well sites or holes that are open on both the top and bottom, with the top opening large enough for a desired cell to pass through and the bottom opening too small for the desired cell to pass through (Weinreb et al., U.S. Pat. No. 5,506,141). The diameters of eukaryotic cells are greater than about 10 μm and those of the smallest prokaryotic cells, genus Mycoplasma, are about 0.15-0.30 μm; microfabrication techniques for manufacturing arrays of well sites or holes to accommodate cells of these diameters are well known (for example, Chu et al. in U.S. Pat. No. 6,044,981 teach methods for making holes or channels having dimensions as small as about 5 nanometers (nm) by employing a sacrificial layer; these dimensions are smaller than the resolution limit of photolithography, currently 350 nm). No currently employed methods of manipulating cells permit making an ordered array of single cells on a planar substrate surface. Further, no method of sorting cells into individual array sites by size exists other than that of controlling physical hole or well size as described by Weinreb et al., supra, to permit cell populations of differing size to enter and be contained in non-planar holes or wells.
The screening of cells is appreciated to initially require a relatively large known number of individual cells (as described for example by Weinreb et al., U.S. Pat. No. 5,506,141), to ensure detection of a particular cell function or characteristic among a population of cells at different life cycle stages and varying in other characteristics. It is also appreciated that the simultaneous delivery of screening and other reagents requires a fluidic nexus between each cell container and its nearest neighbors. Taylor, U.S. Pat. No. 6,103,479, describes a miniaturized cell array method and device for screening cells, comprising cells in physical wells that are microfluidically connected to independent reagent sources by microchannels that can supply fluid reagents to individual or multiple cells arrayed in the physical wells. Such systems may be easily altered to permit tests on individual cells or a large number of cells simultaneously, but they require costly and detailed microfabrication. The site density of such arrays is limited by the need to make individual wells according to demanding physical specifications, such as minimum well wall thickness for physical integrity and additional space for the channels themselves. Thus a need exists for maximizing site density while maintaining high flexibility for assaying populations and subpopulations, as well as for reducing microfabrication time, expense, and cost. A further need exists for microfluidic delivery of reagents to arrayed cells, whether or not contained in physical wells or localized on a planar substrate in virtual wells, without requiring either a corresponding array or individual microfabricated channels to supply each site with a desired reagent.
No method or device is known to exist for manipulating individual cells by ejecting them from a fluid onto a substrate surface without killing the cells. Thus a need exists for a method and corresponding device for ejecting a single cell from a fluid to a chosen surface locale or region, to permit selective ejection for patterning of cells on a surface. Such selective ejection can be used for making arrays and for other applications requiring cell pattering on a surface, such as engineering tissues and the like, or simply for sorting cells.